In the processing of molten metals, it is often necessary to pump molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so-called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so-called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so-called gas injection pump is used. In each of these pumps, a rotatable impeller is disposed, preferably within a volute case, accessible to the molten metal in the vessel. Upon rotation of the impeller within the volute, the molten metal is pumped as desired in a direction permitted by the volute.
In each of the pumps referred to, the impeller is disposed within the volute formed in a base member. Typically, the base member is suspended within the molten metal by means of posts. The impeller is supported for rotation in the base member by means of a rotatable shaft connected to the drive motor with a coupling. The base member includes an outlet passage in fluid communication with the impeller, and upon rotation of the impeller, molten metal is drawn into the volute and an open section of the impeller, where it then is discharged under pressure to the outlet passage.
Molten metal pump designers are generally concerned with efficiency and effectiveness. For a given diameter impeller, pump efficiency is defined by the work output of the pump divided by the work input of the motor. The equally important quality of effectiveness is defined as molten metal flow per impeller revolutions per minute.
Although pumps previously known in the art operate satisfactorily to pump molten metal from one place to another, certain problems have not been completely addressed. Particularly, these problems relate to the effectiveness of the impeller, duration of operability and consistency of performance.
U.S. Pat. No. 4,940,384, herein incorporated by reference, shows a molten metal pump with a cup-like impeller body having vanes and lateral openings for moving molten metal. Although the impeller of this pump transports molten metal, it is prone to clogging by foreign materials such as semi-solids and solids, e.g. drosses, refractory debris, metallic inclusions, etc., (herein after referred to as "particles") contained in the vessel and frequently drawn into the molten metal pump. If a large particle is drawn into the pump, the impeller can be jammed against the volute case, causing catastrophic failure of the pump. Even if catastrophic failure does not occur, small particles eventually clog the lateral openings and degrade the performance of the impeller by reducing the volume of molten metal it can transfer. Accordingly, it is desirable in the art to have an impeller which minimizes clogging, thereby maintaining high efficiency over time and avoiding catastrophic failure.
U.S. Pat. Nos. 3,776,660 and 5,192,193 also teach molten metal impellers, however these designs have more extensive vanes than U.S. Pat. No. 4,940,384. Nonetheless, each of U.S. Pat. Nos. 3,776,660 and 5,192,193 continue to suggest an impeller design having a larger inlet area than outlet area. Accordingly, the problem of clogging is not overcome by these designs. Moreover, it is easy to envision a particle of debris having a size which enters the inlet, adjacent the impeller center, but too large to pass through the narrower passages between the vanes. This particle then bounces around the impeller inlet, reducing flow and degrading the vanes.
Impeller-type equipment without lateral openings has been utilized in molten metal stirring and/or submersion types of devices. U.S. Pat. No. 4,898,367 shows a gas dispersion rectangular block without openings. However, this stirring device does not achieve a directed, forced fluid flow. Particularly, the impeller must be rotatable within a housing to maximize forced flow from the impellers rotation. In addition to block type molten metal agitation devices, vaned circular equipment has been used, see U.S. Pat. No. 3,676,382. Again, however, there is no means for achieving forced directional molten metal flow. Such forced directional molten metal flow is highly necessary in the application of pumping technology to molten metal processing. For example, in a circulation mode, better convectional heat transfer occurs (greater kinetic energy imparted by the pump), and faster melting exists as solid charge materials such as scrap or ingot is mixed more quickly and thoroughly into and with the liquid metal. In a transfer mode, the liquid metal is more strongly directed or redirected into a conveying conduit such as a riser or pipeline for more efficient transfer at a higher rate as a result of such improved forced directional molten metal flow. In a gas injection mode, treatment with gas is more readily achieved with a contained molten metal flux.
In summary, the molten metal treatment art described in the above paragraphs fails to achieve important advantages of the current invention. Particularly, either there is not effective prevention of clogging and/or there is no means to achieve directional forced molten metal flow.
The current invention achieves a number of advantages in directional forced molten metal flow. For example, the impeller of the current pump is not prone to clogging of lateral openings as in the prior pump impellers. Accordingly, catastrophic failure is much less likely to occur and the effectiveness of the impeller operation does not degrade as rapidly over time. The design also achieves high strength by increasing the load area material thickness. Furthermore, the impeller design can be prepared with easy manufacturing processes. Accordingly, the cost of production is reduced and accommodates a wide selection of impeller material, such as graphite or ceramic. Also, the current impeller invention is adaptable to allow optimization as required without large scale manufacturing alteration.